How to: Server-Sent Events with Indy HTTP Server (part 1)

In this article, a server application uses the Indy HTTP server to provide a HTML page which uses SSE to update its content with data sent from the server.

Server-Sent Events (SSE) is a server push technology enabling a client to receive automatic updates from a server via an HTTP connection, and describes how servers can initiate data transmission towards clients once an initial client connection has been established.
Server-sent events, https://en.wikipedia.org/w/index.php?title=Server-sent_events&oldid=1093881969 (last visited Sept. 17, 2022).

Part 1: a very basic demo application

Ingredient #1: a HTML page with Javascript

&lt;!DOCTYPE html&gt;
&lt;html&gt;
	&lt;head&gt;
	&lt;title&gt;SSE example&lt;/title&gt;
	&lt;/head&gt;
	&lt;body&gt;
	&lt;script&gt;
	const evtSource = new EventSource("sse");
		
	evtSource.addEventListener("ping", (event) =&gt; {
	  const newElement = document.createElement("li");
	  const eventList = document.getElementById("list");
	  const time = JSON.parse(event.data).time;
	  newElement.textContent = `ping at ${time}`;
	  eventList.appendChild(newElement);
	});
	&lt;/script&gt;
	&lt;ul id="list"&gt;
	&lt;/ul&gt;
	&lt;/body&gt;
&lt;/html&gt;

The code is based on the article Using server-sent events on MDN

https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events

Ingredient #2: server side code

In the example application, a TIdHTTPServer subclass is used to deliver the HTML document when a browser accesses it.

Note that the server has to keep connections alive, so the Server property KeepAlive must be set to true:

  procedure Test;
  begin
    Server := TMySSEServer.Create;
    try
      Server.KeepAlive := True;
      Server.Startup;
      ReadLn;
    finally
      Server.Free;
    end;
  end;

The DoCommandGet method is overriden and looks like this:

  procedure TMySSEServer.DoCommandGet(AContext: TIdContext;
    ARequestInfo: TIdHTTPRequestInfo; AResponseInfo: TIdHTTPResponseInfo);
  var
    S: TStream;
    C: string;
  begin
    if ARequestInfo.Document = '/sse' then
    begin
       AResponseInfo.ContentType := 'text/event-stream';
       AResponseInfo.CacheControl := 'no-store';
       AResponseInfo.CharSet := 'UTF-8';
      
         C := 'event: ping' + #13
           + Format('data: {"time": "%d"}', [GetTickCount]) 
           + #13#13;

       AResponseInfo.ContentText := C;
       AResponseInfo.ResponseNo := 200;
    end
    else
    begin
      S := TFileStream.Create('index.html', fmOpenRead);
      AResponseInfo.ContentType := 'text/html';
      AResponseInfo.ContentStream := S;
      AResponseInfo.ResponseNo := 200;
    end;
  end;

Output

When the browser navigates to http://localhost, the server will provide the HTML and the embedded JavaScript will start reading data from the address http://localhost/sse (and receive only one event). As specified in the HTML spec for EventSource, the client will retry after some seconds:

Each EventSource object has the following associated with it:
– A reconnection time, in milliseconds. This must initially be an implementation-defined value, probably in the region of a few seconds.
HTML – 9.2 Server-sent events (https://html.spec.whatwg.org/multipage/server-sent-events.html#server-sent-events)

Note: this is just the first step. In the next part, we will implement a continuos data stream.

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